Finally! We can use the viewport compositor for renders! All in Blender 4.0 Alpha as of June 6th, I think.
Also, I just got a new computer and as you see the cpu is relatively beefy. On my old six core intel 8700 this example compositing structure was taking up to a minute. So if you don't have a super strong CPU this is going to be very good news for you.
I feel like I really went all out with that little skit at the beginning. It took me so many takes to lip-sync that line.
0:00 intro skit
0:48 timing old and new compositors
2:31 how to enable it
3:41 rendering animations (fail)
6:24 tweaks needed
6:44 the end